In wind turbine systems known in the art, sudden changes in the velocity of the wind acting on the rotor of the wind turbine cause the tower of the wind turbine to sway. Such oscillation of the wind turbine tower is known to significantly shorten the technical life span of the tower, and also to produce significant mechanical load in the yaw system and gearing.
Such oscillations can be still dramatically increased in cases of variable speed turbines, in which certain rotational speeds can cause oscillations that match the tower resonant frequency.
In order to avoid this adverse effect, systems of the art have been built strong enough to tolerate these extra forces, particularly regarding the tower construction. However, such an approach leads to significant technical overdimensioning and thus extra technical effort, and may also cause logistic problems as to transporting the wind turbine components, since such components may become too large or heavy to economically deliver them to the wind turbine site.
In order to reduce such extra effort, it is known in the art to control the rotational speed of the wind turbine such that the actual rotor speed is kept away from a critical speed, that is, a speed that would produce excessive vibration due to resonance with the tower resonant frequency. However, such systems need to be manually configured, in order to properly specify the critical speed to be avoided. Such manual configuration complicates the process of adding wind turbines to the grid.
U.S. Pat. No. 4,700,081 shows a method and apparatus that performs a speed control of a wind turbine such that a critical speed is avoided. This system increases the rotational speed at a specified rate except in the vicinity of the critical speed. When approaching the critical speed, the rate of increase is reduced to avoid reaching the critical speed. In further scenarios, the change rate is adapted such that the critical speed is quickly passed through to not induce resonant vibrations.